Volume 14, Issue 23 - Special Issue: In Honor of Federico Capasso, A Visionary in Nanophotonics

Nonlinear intersubband polaritonic metasurfaces produce some of the strongest second- and third-order nonlinear optical responses reported for condensed matter systems at infrared frequencies. These metasurfaces are fabricated as two-dimensional arrays of nanoresonators from multi-quantum-well semiconductor heterostructures, designed to produce strong nonlinear responses associated with intersubband transitions. By optimally coupling the optical modes of the nanoresonators to vertically polarized intersubband transitions in semiconductor heterostructures, one can boost the nonlinear response associated with intersubband transitions, make intersubband transitions interact with free-space radiation at normal incidence, and hence produce optically thin flat nonlinear optical elements compatible with free-space optical setups. As a result of the strong nonlinear response in these systems, significant nonlinear conversion efficiencies (>0.1 %) can be attained in deeply subwavelength optical films using modest pumping intensities of only 10–100 kW/cm 2 . Subwavelength metasurface thickness relaxes phase-matching constraints limiting the operation of bulk nonlinear crystals. Furthermore, the amplitude and phase of the nonlinear optical response in intersubband polaritonic metasurfaces can be tailored for a specific pump wavelength and a nonlinear process of interest through the co-optimization of quantum engineering of electron states in semiconductor heterostructures and photonic engineering of the metasurface nanoresonators design. Additionally, an applied voltage can dynamically control the amplitude and phase of the nonlinear optical response at a nanoresonator level. Here, we review the current state of the art in this rapidly expanding field, focusing on nonlinear processes supporting second-harmonic generation, saturable absorption, and optical power limiting.

Polaritons are quantum mechanical superpositions of photon states with elementary excitations in molecules and solids. The light–matter admixture causes a characteristic frequency-momentum dispersion shared by all polaritons irrespective of the microscopic nature of material excitations that could entail charge, spin, lattice or orbital effects. Polaritons retain the strong nonlinearities of their matter component and simultaneously inherit ray-like propagation of light. Polaritons prompt new properties, enable new opportunities for spectroscopy/imaging, empower quantum simulations and give rise to new forms of synthetic quantum matter. Here, we review the emergent effects rooted in polaritonic quasiparticles in a wide variety of their physical implementations. We present a broad portfolio of the physical platforms and phenomena of what we term polaritonic quantum matter. We discuss the unifying aspects of polaritons across different platforms and physical implementations and focus on recent developments in: polaritonic imaging, cavity electrodynamics and cavity materials engineering, topology and nonlinearities, as well as quantum polaritonics.

Photonic device development (PDD) has achieved remarkable success in designing and implementing new devices for controlling light across various wavelengths, scales, and applications, including telecommunications, imaging, sensing, and quantum information processing. PDD is an iterative, five-step process that consists of: (i) deriving device behavior from design parameters, (ii) simulating device performance, (iii) finding the optimal candidate designs from simulations, (iv) fabricating the optimal device, and (v) measuring device performance. Classically, all these steps involve Bayesian optimization, material science, control theory, and direct physics-driven numerical methods. However, many of these techniques are computationally intractable, monetarily costly, or difficult to implement at scale. In addition, PDD suffers from large optimization landscapes, uncertainties in structural or optical characterization, and difficulties in implementing robust fabrication processes. However, the advent of machine learning over the past decade has provided novel, data-driven strategies for tackling these challenges, including surrogate estimators for speeding up computations, generative modeling for noisy measurement modeling and data augmentation, reinforcement learning for fabrication, and active learning for experimental physical discovery. In this review, we present a comprehensive perspective on these methods to enable machine-learning-assisted PDD (ML-PDD) for efficient design optimization with powerful generative models, fast simulation and characterization modeling under noisy measurements, and reinforcement learning for fabrication. This review will provide researchers from diverse backgrounds with valuable insights into this emerging topic, fostering interdisciplinary efforts to accelerate the development of complex photonic devices and systems.

The investigation of interaction of light with various materials on the sub-cycle time scale requires field sampling techniques with incredibly high temporal resolution. Electro-optic sampling (EOS) provides the sub-wavelength resolution both in time and space giving the opportunity for ultrafast microscopy to observe response of electrons and quasiparticles in real time. For the frequencies approaching the petahertz scale, the oscillations of light are incredibly hard to resolve. In particular, EOS has not been demonstrated for wavelengths below 700 nm. In this perspective, we discuss the potential extension of EOS to cover complete visible spectrum and the impact that it can give to the nanophotonics and material science. Specifically, we describe how the ultrafast dynamics of quasiparticles in some 2D materials can be tracked using the space-resolved EOS.

Tailoring structured beams with varying lightwave properties along the longitudinal dimension has recently gained much interest. This paper gives a perspective on the advances of longitudinally structured beams, their potential applications, and future opportunities.

Augmented and virtual reality (AR/VR) is transforming how humans interact with technology in a wide range of fields and industries, from healthcare and education to entertainment. However, current device limitations have impeded wider integration. Tunable holographic metasurfaces represent a promising platform for revolutionizing AR/VR devices by precisely controlling light at the subwavelength scale. This article examines current challenges and opportunities from both the AR/VR and holographic metamaterial perspectives and explores how improvements to state-of-the-art approaches can address these challenges. In particular, we propose a focus on easily manufacturable and broadly electrically tunable metasurface technologies including liquid crystal integration and excitonic tuning in 2D materials. Advanced metasurface optimization techniques including machine learning will also be crucial for exploring the large design space.

Polarization control and switchability are among the most unique features of “metasurfaces” as compared with diffractive optics technologies of the past. Here, we review how the polarization control afforded by the advent of present-day metasurfaces compares to diffractive elements of previous decades, clarifying from a functional perspective what is new, and what is not.

Metasurfaces have been widely exploited in imaging and sensing, holography, light–matter interaction, and optical communications in free space and on chip, thanks to their CMOS compatibility, versatility and compact form. However, as this technology matured from novelty to performance, stringent requirements on diffraction efficiency, scalability, and complex light control have also emerged. For instance, the limited thickness of single-layer meta-optics poses fundamental constraints on dispersion engineering and lossless transmission over large-scale devices, whereas in-plane symmetry limits the polarization transformations that can be realized. Cascaded and multi-layer flat optics can alleviate these constraints, offering new possibilities for realizing high-efficiency devices, full polarization control, and achromatic response. In this perspective, recent advances in multi-layer metasurfaces including inherent challenges and opportunities will be discussed. Compound meta-optics hold the promise for enabling complex optical systems with enhanced performance and unprecedented functionality for a diverse set of applications in sensing, imaging, high-capacity communications, and beyond.

In this opinion, we describe the potential of an emerging class of flat optics known as “nonlocal metasurfaces” to manipulate light in both real space and momentum space. While the ultimate form of a conventional “local” metasurface can be viewed as a universal generator of any desired waveform from a fixed input wavefront, the ultimate form of a nonlocal metasurface would instead act as a universal “map” from a given set of input waveforms to a set of orthogonal output waveforms. Here, we discuss how this implies four-dimensional information capacity, drastically enhancing information density compared to local metasurfaces. We discuss a framework using scattering matrices and a nonlocal generalized Snell’s law to describe nonlocal metasurfaces. We comment on the potential, progress, and practicality of this ambitious vision, suggesting limitations and next steps.

Light encodes information in multiple degrees of freedom (e.g., frequency, amplitude, and phase), enabling high-speed, high-bandwidth communication through fiber optics. Unlike classical light, quantum light (single or entangled photons) can transmit quantum states over long distances without loss of coherence, thereby coherently interconnecting quantum nodes for distributed quantum entanglement. Quantum light sources are critical for developing scalable quantum networks aimed at distributed quantum computing, quantum teleportation, and secure quantum communications. However, existing quantum light sources suffer from limited integrability, insufficient spectral and spatial tunability, and inefficiencies in achieving mass-produced, deterministic, on-demand quantum light generation. These limitations significantly hinder progress toward direct, on-chip integration with quantum processing units and detectors – an essential step toward scalable quantum networks. Resonant metasurfaces that leverage photonic modes – such as Mie resonances, guided-mode resonances, or symmetry-protected bound states in the continuum – offer strong spatial and temporal confinement of electromagnetic fields, characterized by high quality factors and small mode volumes. These metasurfaces greatly enhance linear and nonlinear light-matter interactions, making them ideal for efficient on-chip quantum light generation and manipulation. Here, we describe recent advances in nanoscale quantum light sources and quantum photonic state manipulation enabled by resonant metasurfaces. We also provide an outlook on next-generation miniaturized quantum light sources achievable through materials innovations in quantum emitters, the co-design of resonant metasurfaces, and ultimately, the heterogeneous integration of emerging layered van der Waals materials with resonant metasurfaces.

The emergence of metasurfaces has enabled lightweight, compact imaging with degrees of freedom which previously required complex optical setups to achieve, such as polarization, wave vector, and spectrum. To date, most metasurface-enabled imaging systems have thus far been ‘passive’, and therefore subject to fundamental information and thickness limits set by the coupling of light to their sensor arrays. We discuss the use of active metasurfaces in low form-factor and low pixel-count imaging systems and introduce a prototypical lensless imaging system concept which employs an active metasurface as a high-frequency, continuously tunable amplitude and phase modulation aperture, coupled to a discrete single-pixel detector. We analyze the scalability of such a platform and computationally demonstrate that a scalable ‘perimeter-control’ addressing architecture – in which a M × N rectangular array of scattering elements is addressed by only M + N voltages – is sufficient for image collection, even when scatterers exhibit limited 272 ◦ phase control, and undesired amplitude variations. We also address fundamental limits in information collection, image aberrations, and signal-to-noise ratio, highlighting key advantages, limitations, and trade-offs for active metasurface imaging. We generalize our discussion to other active metasurface-enabled imaging configurations and applications. Finally, we consider promising active metasurface material platforms with an outlook towards new directions to enable high-efficiency imaging.

Nanogap cavity metasurfaces – an array of metallic nanoparticles separated from a metal plane by a nanometer-scale dielectric material – can manipulate electromagnetic waves across a wide wavelength range. Through this, they can profoundly modify the optical processes of molecules and materials relevant to quantum communications, photocatalysis, and optoelectronics. Interactions between nanocavities and light, however, require overlap between the cavity resonance and the energy of the incident photon or optical transition, demanding labor-intensive fabrication of bespoke metasurfaces for each desired application. Here, we dynamically tune the resonance wavelength of nanogap cavity metasurfaces by modulating the refractive index of the surrounding medium using solvents. We achieve precise, reversible, and broadband resonance control for narrow nanogap cavity resonances (full width half max <500 nm) over a range of 1–5 µm, while maintaining high absorption efficiency (60–98 %). Resonance tuning up to 300 nm for a single metasurface was achieved by changing the dielectric environment from air to solvents with controlled refractive indexes n = 1.3–1.7 without any discernable metasurface degradation. This opens new possibilities for applications in optical sensing with significantly increased nanofabrication tolerances, such as tunable photonic devices and adaptive optical systems, where precise control over light–matter interactions is critical.

The modification of light’s trajectory after refracting through a boundary separating two media is a ubiquitous phenomenon in nature. The laws governing such refraction/reflection, known today as the Snell–Descartes laws of reflection and refraction, were established over four centuries ago and have since become foundational to the field of classical optics. Presently, with the advent of nano-photonic technology, a generalized version of these laws has been developed and implemented, vastly broadening the breadth of light manipulation methods. Despite their popularity, however, a simple and accessible derivation of the Snell–Descartes laws is still lacking, and their generalization is still largely missing from the physics curricula. Here, we use simple analogies between light’s refraction and reflection and other a priori unrelated radiating wave systems, namely, shock waves, water wakes, and Cherenkov radiation to derive both the classical and generalized Snell–Descartes laws, relying solely on simple and intuitive arguments. The basis of the derivation considers the excitation of a surface perturbation, induced by light incident at an angle on a boundary, that propagates at a velocity exceeding the phase velocity of light in the medium. The perturbation thereafter acts as a radiative source that reflects and refracts light away from the interface, at angles satisfying the classical Huygens interference condition. These derivations are meant to be accessible to a broad range of readers, including students of all levels, middle/high school teachers, and beyond.

High refractive index dielectric materials like silicon rich nitride (SRN) are critical for constructing advanced dielectric metasurfaces but are limited by transparency and complementary metal oxide semiconductor (CMOS) process compatibility. SRN’s refractive index can be adjusted by varying the silicon to nitride ratio, although this increases absorption, particularly in the blue spectrum. Dielectric metasurfaces, which utilize the material’s high dielectric constant and nano-resonator geometry, experience loss amplification due to resonance, affecting light reflection, light transmission, and quality factor. This study explores the impact of varying the silicon ratio on structural color applications in metasurfaces, using metrics such as gamut coverage, saturation, and reflection amplitude. We found that a higher SRN ratio enhances these metrics, making it ideal for producing vivid structural colors. Our results show that SRN can produce a color spectrum covering up to 166 % of the sRGB space and a resolution of 38,000 dots per inch. Fabricated samples vividly displayed a parrot, a flower, and a rainbow, illustrating SRN’s potential for high-resolution applications. We also show that SRN can provide a better CIE diagram coverage than other popular metasurfaces materials. These findings highlight the advantages of SRN for photonic devices, suggesting pathways for further material and application development.

Silicon nitride (SiN) formed via low pressure chemical vapor deposition (LPCVD) is an ideal material platform for on-chip nonlinear photonics owing to its low propagation loss and competitive nonlinear index. Despite this, LPCVD SiN is restricted in its scalability due to the film stress when high thicknesses, required for nonlinear dispersion engineering, are deposited. This stress in turn leads to film cracking and makes integrating such films in silicon foundries challenging. To overcome this limitation, we propose a bilayer waveguide scheme comprised of a thin LPCVD SiN layer underneath a low-stress and low-index PECVD SiN layer. We show group velocity dispersion tuning at 1,550 nm without concern for film-cracking while enabling low loss resonators with intrinsic quality factors above 1 million. Finally, we demonstrate a locked, normal dispersion Kerr frequency comb with our bilayer waveguide resonators spanning 120 nm in the c-band with an on-chip pump power of 350 mW.

Recent years have witnessed significant developments in the study of nonlinear properties of various materials at the nanoscale. Often, experimental results on harmonic generation are reported without the benefit of suitable theoretical models that allow assessment of conversion efficiencies compared to the material’s intrinsic properties. Here, we report experimental observations of even and odd harmonics up to the 7 th, generated from a suspended subwavelength silicon film resonant in the UV range at 210 nm, the current limit of our detection system, using peak power densities of order 3 TW/cm 2 . We also highlight the time-varying properties of the dielectric function of silicon, which exhibits large changes under intense illumination. We explain the experimental data with a time domain, hydrodynamic-Maxwell approach broadly applicable to most optical materials. Our approach accounts simultaneously for surface and magnetic nonlinearities that generate even optical harmonics , as well as linear and nonlinear material dispersions beyond the third order to account for odd optical harmonics , plasma formation, and a phase locking mechanism that makes the generation of high harmonics possible deep into the UV range, where semiconductors like silicon start operating in a metallic regime.

Space-time–modulated systems have attracted significant interest over the past decade due to their ability to manipulate electromagnetic waves in unprecedented ways. Here, we introduce a new type of space-time–modulated structure, the space-time wedge , consisting of two interfaces moving at different velocities, which results in either closing or opening wedges. Using moving boundary conditions, we derive closed-form solutions for the scattering of electromagnetic waves in such a wedge and leverage these solutions to unveil the underlying physics, including multiple space-time scattering and Doppler shifting. The space-time wedge holds potential for various optical and photonic applications.

We perform an experimental two-color high-order harmonic generation study in argon with the fundamental of an ytterbium ultrashort pulse laser and its second harmonic. The intensity of the second harmonic and its phase relative to the fundamental are varied while keeping the total intensity constant. We extract the optimum values for the relative phase and ratio of the two colors which lead to a maximum yield enhancement for each harmonic order in the extreme ultraviolet spectrum. Within the three-step model, the yield maximum can be associated with a flat electron return time versus return energy distribution. An analysis of different distributions allows to predict the required relative two-color phase and ratio for a given harmonic order, total laser intensity, fundamental wavelength, and ionization potential.

Twistronics is studied intensively in twisted 2D heterostructures and its extension to trilayer moiré structures has proven beneficial for the tunability of unconventional correlated states and superconductivity in twisted trilayer graphene. Just recently, the concept of twistronics has been applied to plasmonic lattices with nontrivial topology, demonstrating that bilayer moiré skyrmion lattices harbor multi-skyrmion textures called skyrmion bags. Here, we explore the properties of plasmonic trilayer moiré superlattices that are created by the interference of three twisted skyrmion lattices. More specifically, we explore the properties of periodic superlattices and their topological invariants. We also demonstrate that twisted trilayer skyrmion lattices harbor the same skyrmion bags as twisted bilayer skyrmion lattices. We quantify the robustness of these skyrmion bags by the stability of their topological numbers against certain disturbance fields that leads to experimental designs for topological textures with maximum robustness.

The COVID-19 pandemic has profoundly impacted global economies and healthcare systems, revealing critical vulnerabilities in both. In response, our study introduces a sensitive and highly specific detection method for cDNA, leveraging Luminescence Resonance Energy Transfer (LRET) between upconversion nanoparticles (UCNPs) and gold nanoparticles (AuNPs), and achieves a detection limit of 242 fM for SARS-CoV-2 cDNA. This innovative sensing platform utilizes UCNPs conjugated with one primer and AuNPs with another, targeting the 5′ and 3′ ends of the SARS-CoV-2 cDNA, respectively, enabling precise differentiation of mismatched cDNA sequences and significantly improving detection specificity. Through rigorous experimental analysis, we established a quenching efficiency range from 10.4 % to 73.6 %, with an optimal midpoint of 42 %, thereby demonstrating the superior sensitivity of our method. Our work uses SARS-CoV-2 cDNA as a model system to demonstrate the potential of our LRET-based detection method. This proof-of-concept study highlights the adaptability of our platform for future diagnostic applications. Instrumental validation confirms the synthesis and formation of AuNPs, addressing the need for experimental verification of the preparation of nanomaterial. Our comparative analysis with existing SARS-CoV-2 detection methods revealed that our approach provides a low detection limit and high specificity for target cDNA sequences, underscoring its potential for targeted COVID-19 diagnostics. This study demonstrates the superior sensitivity and adaptability of using UCNPs and AuNPs for cDNA detection, offering significant advances in rapid, accessible diagnostic technologies. Our method, characterized by its low detection limit and high precision, represents a critical step forward in developing next-generation biosensors for managing current and future viral outbreaks. By adjusting primer sequences, this platform can be tailored to detect other pathogens, contributing to the enhancement of global healthcare responsiveness and infectious disease control.

We present a photonic mode converter based on a grating structure, modeled and inversely designed by deep neural networks. The neural network maps the physical parameters of the grating to the grating responses, i.e., complex scattering parameters representing the reflected modes from the grating structure. We design different neural networks to output the magnitudes and the phases of the scattering parameters associated with the multiple reflected modes. Following the training process, we use the trained networks to perform inverse design of the grating based on the desired magnitudes of the scattering parameters. The inverse design effort provides a full control on the magnitudes and the phases of the reflected modes from the mode converter. Our techniques help in creating a rich landscape of multiple interfering waves that provide higher control on optical near fields, complex resonators, and their relevant nanophotonic applications.

Recent studies have shown that low-symmetry conductors under static electric bias offer a pathway to realize chiral gain, where the non-Hermitian optical response of the material is controlled by the spin angular momentum of the wave. In this work, we uncover the topological nature of chiral gain and demonstrate how a static electric bias induces topological bandgaps that support unidirectional edge states at the material boundaries. In our system, these topological edge states consistently exhibit dissipative properties. However, we show that, by operating outside the topological gap, the chiral gain can be leveraged to engineer boundary-confined lasing modes with orbital angular momentum locked to the orientation of the applied electric field. Our results open new possibilities for loss-compensated photonic waveguides, enabling advanced functionalities such as unidirectional, lossless edge-wave propagation and the generation of structured light with intrinsic orbital angular momentum.

Plasmonic biosensors are powerful platforms for detecting various types of analytes. Specifically, surface-enhanced Raman spectroscopy (SERS) can enable label-free and selective detection. Shiga toxin-producing Escherichia coli (STEC) represents zoonotic pathogens that cause severe diseases, such as hemolytic uremic syndrome (HUS), the most important cause of acute renal failure in children. To date, there are no effective therapies for STEC infection, and the available diagnostic methods are complex and inconclusive. Here, a novel nanopattern fabricated by electron beam lithography with remarkable plasmonic properties is employed as SERS substrate for realizing the specific recognition of Stx1a, Stx2a, and of a third variation of the latter. A limit of detection (LOD) of 6.8 pM for Stx1a and 2 pM for Stx2a was achieved. Our approach supported using the principal component analysis (PCA) appears to be a valid alternative to conventional methods, allowing real-time and fast in situ analysis.

In this paper, we model the interaction of a quantum emitter with a finite-size dispersive dielectric object in unbounded space within the framework of macroscopic quantum electrodynamics, using the modified Langevin noise formalism. The quantized electromagnetic field consists of two contributions: the medium-assisted field, which accounts for the electromagnetic field generated by the noise polarization currents of the dielectric, and the scattering-assisted field, which takes into account the electromagnetic field incoming from infinity and scattered by the dielectric. We show that the emitter couples to two distinct bosonic reservoirs: a medium-assisted reservoir and a scattering-assisted reservoir, each characterized by its own spectral density. We then use emitter-centered modes to reduce the degrees of freedom of both reservoirs. Eventually, we identify the conditions under which the electromagnetic environment composed of these two reservoirs can be effectively replaced by a single bosonic reservoir so that the reduced time evolution of the quantum emitter remains unchanged. In particular, when the initial states of the medium- and scattering-assisted reservoirs are thermal quantum states at the same temperature, we find that a single bosonic bath with a spectral density equal to the sum of the medium- and scattering-assisted spectral densities is equivalent to the original electromagnetic environment.

Stimulated by seminal works on generalized reflection and refraction laws, metasurfaces have evolved to a highly promising research direction, as they allow a multitude of different functionalities by optically thin wave-control elements/structures. Among them, structures functioning in the low THz regime are of extensive research interest, due to their high potential in communication and sensing applications. In this paper we propose a simple THz metasurface design that exhibits ideally perfect cross-polarized reflection for linear polarization and perfect helicity-preserving reflection with a geometric (Pancharatnam–Berry) phase for circular polarization, without the presence of any back-reflector. Numerical calculations demonstrating the structure response are justified by analytical models, which provide physical insights on this response. The designed metasurfaces are fabricated using the direct laser writing 3D-printing technology, metallized with electroless silver plating, and are characterized by THz time domain spectroscopy, with the experimental results validating the corresponding theoretical ones. Applications including beam steering and focusing, exploring the Pancharatnam–Berry phase, are also demonstrated numerically. Besides those applications, the helicity preserving mirror response of our metasurfaces can be valuable, among others, also in molecular chirality sensing applications, an issue that is also highlighted here.

The phenomenon of superoscillations is of great interest in microscopy, antenna design, and material sciences. This phenomenon has been generalized and has given rise to the concept of supershift, which is a far reaching extension that applies to functions that may present discontinuous derivatives. From this perspective, this is a notion that might have significant applications. This paper will provide an up to date report on the complex connections between the concept of supershift and that of analyticity.

Controlling and manipulating radiative heat transfer remains a pivotal challenge in both scientific inquiry and technological advancement, traditionally tackled through the precise geometric design of metastructures. However, geometrical optimization cannot break the inherent shackles of local modes within individual meta-atoms, which hinders sustained progress in radiative heat transfer. Here, we propose a comprehensive strategy based on interatomic displacement to achieve superior heat transfer performance while obviating the need for increasingly complex structural designs. This meta-atomic displacement strategy enables a shift from quasi-isolated localized resonances to extended nonlocal resonant modes induced by strong interactions among neighboring meta-atoms, resulting in a radiative heat conductance that surpasses other previously reported geometrical structures. Furthermore, this meta-atomic displacement strategy can be seamlessly applied to various metastructures, offering significant implications for advancing thermal science and next-generation energy devices.

Finite-size effects occurring for quasi-bound states in the continuum (qBICs) formed in symmetric and asymmetric all-dielectric waveguide grating couplers are investigated using numerical simulations for different configuration parameters. We find that the beam size plays a crucial role in the formation of additional qBIC resonances originating in forbidden for plane-wave incidence BIC resonances, whose manifestation is strongly influenced by the beam divergence. Another parameter is the strength of Bragg reflection, determining the spatial extension of a distributed Bragg resonator (DBR): in configurations with strong Bragg reflection, the excited and Bragg reflected counterpropagating waveguide modes are confined within the footprint of the incident beam. Conversely, the DBR formed in configurations with weak Bragg reflection extends well beyond the incident beam footprint, resulting in high sensitivity of the system response to the grating width and incident beam position. We believe that our findings provide valuable insights for optimizing photonic devices that leverage qBIC resonances in finite-sized structures under realistic excitation conditions.

We predict the existence of surface plasmon polaritons at the interface between a metal and a periodically modulated dielectric medium, and find an unusual multi-branched dispersion curve of surface and bulk modes. The branches are separated by momentum gaps indicating intense amplification of modes, and display high and low group velocity ranging from zero to infinity at short wavelengths. We simulate how these SPP modes are formed by launching a properly engineered laser beam onto the metallic interface and examine their space-time evolution. The amplification of the surface plasmons at the interface of a photonic time-crystal offers a path to overcome plasmonic losses, which have been a major challenge in plasmonics.

Optical Stokes skyrmions represent an emerging class of structured light characterized by intricate topological polarization textures in the beam’s transverse plane. Traditional methods for generating Stokes skyrmions rely on bulky optical setups, driving significant interests in compact, single-device solutions. However, existing approaches fail to ensure propagation-invariant topology, an imperative requirement for advancing applications in this field. In this paper we address this fundamental challenge with a metasurface design based on structural birefringence and geometric phase which manipulates light in dynamic phase iso-curves, achieving arbitrary co-polarization to cross-polarization conversion while maintaining a constant dynamic phase. This design enables propagation-invariant topological features of optical skyrmions produced by a single generation device. Our framework offers a compact platform for shaping topologically stable optical skyrmions, which may stimulate their applications for long-range optical information transfer.

We implement a paraxial azimuthally-radially polarized beam (ARPB), a novel class of structured light beams that can be optimal chiral (OC), leading to maximum chirality density at a given energy density. By using vectorial light shaping techniques, we successfully generated a paraxial ARPB with precise control over its features, validating theoretical predictions. Our findings demonstrate the ability to finely adjust the chirality density of the ARPB across its entire range by manipulating a single beam parameter. Although our experimental investigations are primarily focused on the transverse plane, we show that fields whose transverse components satisfy the optimal chirality condition are optimally chiral in all directions, and our results highlight the promising potential of OC structured light for applications in the sensing and manipulation of chiral particles. We show that helicity density is more general than the concept of handedness. This work represents a significant advancement toward practical optical enantioseparation and enantiomer detection at the nanoscale.

Due to their attractive optical properties, 2D MXenes have garnered interest in nanophotonic and optoelectronic applications. However, tuning their properties typically requires the iterative synthesis of MXenes with a specific set of properties, such as the absorption band position, electronic conductivity, and dielectric constant. We demonstrate how to tailor the optical properties of MXene thin films over a broad 1500-nm wavelength range by mixing different ratios of highly conductive Ti 3 C 2 T x with poorly conductive Nb 2 CT x . By changing the MXene film composition, the epsilon-near-zero (ENZ) point, where the optical properties transit from dielectric to metallic, was varied in the spectral range from 1.1 to 2.6 µm. Additionally, we observed a reduction in absorption in some compositions compared to the absorption of the pure MXene films. Compared to other methods, this approach enables simple and continual tuning of MXene optical properties without requiring multiple time-consuming synthesis steps.

Digital signal processing is the cornerstone of several modern-day technologies, yet in multiple applications it faces critical bottlenecks related to memory and speed constraints. Thanks to recent advances in metasurface design and fabrication, light-based analog computing has emerged as a viable option to partially replace or augment digital approaches. Several light-based analog computing functionalities have been demonstrated using patterned flat optical elements, with great opportunities for integration in compact nanophotonic systems. So far, however, the available operations have been restricted to the linear regime, limiting the impact of this technology to a compactification of Fourier optics systems. In this paper, we introduce nonlinear operations to the field of metasurface-based analog optical processing, demonstrating that nonlinear optical phenomena, combined with nonlocality in flat optics, can be leveraged to synthesize kernels beyond linear Fourier optics, paving the way to a broad range of new opportunities. As a practical demonstration, we report the experimental synthesis of a class of nonlinear operations that can be used to realize broadband, polarization-selective analog-domain edge detection.

The confinement and manipulation of Janus particles have recently garnered significant interest due to their potential applications in fields such as nanotechnology and biophysics, where, under specific circumstances, they can act as microengines and drug carriers. However, the dynamics of Janus particles mostly rely on chemical reactions or thermal gradients, limiting their precision application. To tackle these limitations, we propose the 3D manipulation of Janus particles using focused cylindrical vector beams with a doughnut shaped intensity profile above the focal spot. In particular, we study the behaviour, orientation and manipulation of different highly reflective Janus particles composed of silica or polystyrene with a gold cap in the presence of optical potentials generated by focused cylindrical vector beams. Where the radiation pressure predominantly affects the gold cap rather than the bare particle body of the particle. We demonstrated the potential of the proposed levitation technique for controlling a wide range of Janus particles and real-life complex objects with high reflectivity.

We synthesized crystalline films of neodymium nickel oxide (NdNiO 3 ), a perovskite quantum material, switched the films from a metal phase (intrinsic) into an insulator phase (electron-doped) by field-driven lithium-ion intercalation, and characterized their structural and optical properties. Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) showed that the intercalation process resulted in a gradient of the dopant concentration along the thickness direction of the films, turning the films into insulator–metal bilayers. We used variable-angle spectroscopic ellipsometry to measure the complex refractive indices of the metallic and insulating phases of NdNiO 3 . The insulator phase has a refractive index of n ∼ 2 and low absorption in the visible and near-infrared, and analysis of the complex refractive indices indicated that the band gap of the insulating phase is roughly 3–4 eV. Electrical control of the optical band gap, with corresponding large changes to the optical refractive indices, creates new opportunities for tunable optics.

The combination of a tunable quantum cascade laser (QCL) and plasmonic mid-infrared (MIR) metasurface is a powerful tool enabling high-content microscopy of hydrated cells using the vibrational contrast of their constituent biomolecules. While the QCL provides a high-brightness source whose frequency can be rapidly tuned to that of the relevant molecular vibration, the metasurface is used to overcome water absorption of MIR light. Here we employ the resulting metasurface-enabled inverted reflected-light infrared absorption microscopy (MIRIAM) tool for non-destructive monitoring of the vital process of de novo lipogenesis (DNL), by which fat tissue cells (adipocytes) synthesize fatty acids from glucose and store them inside lipid droplets. Using 13 C-labeled glucose as a metabolic probe, we produce spatially- and temporally-resolved images of 13 C incorporation into lipids and proteins, observed as red-shifted vibrational peaks in the MIR spectra. These findings demonstrate MIRIAM’s capability for studying metabolic pathways with molecular specificity, offering a powerful platform for metabolic imaging.

Polarization is a fundamental property of light that can be engineered and controlled efficiently with optical metasurfaces. Here, we employ chiral metasurfaces with monoclinic lattice geometry and achiral meta-atoms for resonant engineering of polarization states of light. We demonstrate, both theoretically and experimentally, that a monoclinic metasurface can convert linearly polarized light into elliptically polarized light not only in the linear regime but also in the nonlinear regime with the resonant generation of the third-harmonic field. We reveal that the ellipticity of the fundamental and higher-harmonic fields depends critically on the angle of the input linear polarization, and the effective chiral response of a monoclinic lattice plays a significant role in the polarization conversion.

Optical trapping, also known as optical tweezing or optical levitation, is a technique that uses highly focused laser beams to manipulate micro- and nanoscopic particles. In optical traps driven by high-energy pulses, material non-linearity can result in unusual opto-mechanical effects, such as displaced equilibrium points. However, existing theoretical models of non-linear optical force on small particles consider smooth material dependence on the incident field strength alone, and not the feedback between the particle permittivity and internal field strength, which is, in turn, a function of the permittivity. The hysteresis effects of optical bistability in pulsed optical traps, therefore, elude existing optical force models. Here, we investigate a bistable optical trap, set up by counter-propagating ultrashort pulses, in which the optical force exerted on a particle depends not only on the field at its current location but on the historic trajectory of the particle in the trap. The developed formalism will be important for designing optical traps and nanoparticle manipulation in pulsed field for various applications, including potentially time crystal demonstrations.

The Hong–Ou–Mandel (HOM) effect, an effective two-photon interference phenomenon, is a cornerstone of quantum optics and a key tool for linear optical quantum information processing. While the HOM effect has been extensively studied both theoretically and experimentally for various photonic quantum states, particularly in the spectral domain, detailed overviews of its behaviour for structured photons – those with complex spatial profiles – under arbitrary spatial mode measurement schemes are still lacking. This tutorial aims to fill this gap by providing a comprehensive theoretical analysis of the HOM effect for structured photons, including an arbitrary mode projection on quantum interference outcomes. The tutorial also provides analytical, closed-form expressions of the HOM visibility under different measurement conditions.

Forty-five years after the initial attempts – first by Efimov–Khitrov in 1979, then by Brendel–Bormann in 1992 – we present a comprehensive, causal, and physically consistent framework for modeling the dielectric function with inhomogeneous (non-Lorentzian) broadening, where scattering becomes frequency- or time-dependent. This theoretical framework is based on spectral diffusion, described in the frequency domain by a complex probability density function and in the time domain by a matching characteristic function. The proposed approach accurately models the lineshapes resulting from multiple broadening mechanisms and enables the retrieval of intrinsic homogeneous linewidths as well as inhomogeneous disorder-controlled material dispersion features. To implement the new general dispersion function in time-domain Maxwell solvers, we have designed a constrained minimax-based semi-analytical approximation method (MiMOSA) that generates the shortest possible numerical stencils for a given approximation error. Application examples of exact and approximate MiMOSA models include the Gauss–Lorentz oscillator, Gauss–Debye relaxation, and Gauss–Drude conductivity. Although this study primarily focuses on the optical domain, the resulting models, which account for the Doppler shift, are equally applicable to other wave propagation phenomena in disordered dispersive media in a broad range of areas, including acoustics, magnonics, astrophysics, seismology, plasma, and quantum technologies.

Arrays of photodetector-based pixel sensors are ubiquitous in modern devices, such as smart phone cameras, automobiles, drones, laptops etc. Two-dimensional (2D) material-based photodetector arrays are a relevant candidate, especially for applications demanding planar form factors. However, shortcomings in pixel density and prototyping without cross contamination limit technology adoption and impact. Also, while 2D material detectors offer high absorption, graphene’s closed bandgap results in undesirably high dark currents. Here, we introduce the experimental demonstration of dense planar photodetector arrays. We demonstrate a micrometer-narrow pitched 2D detector pixels and show this approach’s repeatability by verifying performing of a 16-pixel detector array. Such dense and repeatable detector realization is enabled by a novel, selective, contamination-free 2D material transfer system, that we report here in automated operation. The so realized photodetectors responsivity peaks at a high 0.8 A/W. Furthermore, we achieve uniform detector performance via bias voltage tuning calibration to maximize deployment. Lastly, we demonstrate 2D arrayed photodetectors not only on a silicon chip platform but verify array performance on flexible polymer substrates. Densley-arrayed, flat, bendable, and uniform performing photodetector pixels enable emerging technologies in the space where lightweight and reliable performance is required, such as for the smart phone and emerging VR/AR markets, but also for smart gadgets, wearables, and also for size-weight-power-constrained aviation and space platforms.

Spatial inhomogeneity, temporal modulation, and engineered anisotropy of parameters of electromagnetic media offer numerous opportunities for manipulating light–matter interaction over the past decades. Here, we investigate a scenario in which we deal with the temporal interface, hyperbolic anisotropy in the form of layered structures, and frequency dispersion. We theoretically investigate how a monochromatic uniform plane wave – propagating in an unbounded, homogeneous, isotropic dielectric medium – undergoes changes due to the rapid temporal variation of such medium into a hyperbolic dispersive medium formed by the stack of thin metal–dielectric bilayers, in which the metal follows the lossless Drude dispersion and the dielectric is assumed to be dispersionless. We corroborate our analytical results by numerical simulations. We observe several interesting phenomena, such as conversion of the original frequency into three pairs of frequencies, resulting in three sets of forward (FW) and backward (BW) waves. We present the amplitudes and the time-averaged Poynting vectors for such FW and BW waves and discuss some of the salient features of such temporal interface.

The measurement of the cavity dispersion is a key ingredient in the characterization of a laser frequency comb due to its fundamental role in predicting the multimode laser dynamics. In this work, we discuss the measurement of the modal refractive index dispersion in quantum cascade lasers obtained through the analysis of their sub-threshold luminescence measured using Fourier-transform spectrometers. In particular, we demonstrate the effects of apodization and zero-padding. Finally, we use selected examples to show the effects of different cold-cavity dispersion profiles on the lasing state of the laser. Our results provide a comprehensive guide for determining the cavity dispersion of Fabry–Pérot quantum cascade lasers, which is essential for designing high-performance frequency combs.

Metasurfaces composed by arrays of coupled plasmonic resonators have attracted tremendous attention due to their extraordinary abilities to manipulate electromagnetic (EM) waves. However, existing theories for such systems are either empirical with model parameters obtained by fitting with simulations, or can only be applied to high-frequency systems where metals exhibit finite permittivity. Here, we extend our recently established leaky-eigenmode (LEM) theory to the microwave regime where metals exhibit infinite permittivity, with all parameters directly computable without fitting procedures. After validating our theory with both simulations and experiments on a benchmark metasurface, we illustrate how to utilize the theory to guide designing microwave metasurfaces with freely tailored line-shapes, including particularly the generation of a bound state in the continuum. All theoretical predictions are verified by experiments and simulations. Our study provides a powerful tool to guide designing functional microwave meta-devices for various applications.

Exciton–polariton condensation in optical resonators is a fascinating pathway to realise ultra-coherent states of light, underpinned by Bose–Einstein quasiparticles. Bearing inherently non-radiative nature, bound states in continuum (BIC) have proven to be an excellent platform to achieve polariton condensation. Here, we report exciton–polariton condensation in a solution-processed perovskite thin film hybridized with a silicon BIC metasurface. Thanks to the high quality factor of the BIC, the polariton condensation exhibits low fluence threshold, narrow linewidth, and large spatial and temporal coherence. These results demonstrate the feasibility of integration of perovskite polaritonic devices in scalable silicon photonic platforms.

Quantum cascade laser (QCL) harmonic state frequency combs (HFCs) have recently attracted considerable interest for applications ranging from the generation of tones of high spectral purity, high data rate wireless communication networks, radiofrequency arbitrary waveform synthesis, and for fundamental light-matter investigations in quantum physics. However, a detailed knowledge of the nature of the electronic and thermal energy distribution in these devices is of paramount importance for the refinement of their thermal management and quantum efficiency, which are key to the widespread adoption of QCL HFC technology in a new generation of integrated optical quantum platforms. Here, we perform a comparative study of the thermal and electronic properties of Fabry–Perot and micro-ring HFC QCLs, operating in the terahertz frequency range, using micro-probe band-to-band photoluminescence. By monitoring the lattice temperature and the electron cooling above the threshold for stimulated emission, we extract the device thermal resistances and the internal quantum efficiencies, highlighting the key role of the resonator architecture.

Space heating and cooling account for approximately 15 % of the world’s energy consumption, underscoring the pressing need for improved thermal management. Macroscopic temperature regulation can be significantly optimized by improving radiative heat control, in particular through radiative cooling in summer and sunlight capture for heating in winter. These processes are typically tailored independently and thereby remain passive. In this article, we propose thermally smart windows with radiative properties that adapt to a building’s heating and cooling demands in a tunable manner. To achieve this, one ought to control, simultaneously, the window’s response to near- and mid-infrared (IR) radiation for solar heating and radiative cooling, respectively. We propose device architectures to realize such operations using phase-change materials and liquid crystals. Compared to conventional silica glass, the proposed architectures may reduce the energy demand of buildings at the latitude of Barcelona by more than 40 %, showcasing the potential of tunable materials for radiative thermal management in the energy transition. We discuss that significant promise lies in the development of materials that can warrant near-unity modulation of NIR reflectance, which should be the key property to reach as much as 64 % reduction in energy consumption.